That's pretty cool..
Injections to the economy (a bit of a play on the real life injections)
-Government (i.e. Admin) expenditure
-Inward Investment (other forums investing because they can make more money in your forum?)
-Exports (selling off forum members, haha)
So money would need to be lost as well:
-taxation (certain forums taxed more heavily than others)
-Saving (users might want to save money knowing the intrest rates are high and they can get a good return)
-imports (users from other forums

)